Finding the nudi(s) that eat hydroids is very difficult. I had hydroids take over my tank, over 4 different kinds. I tore down the system and I am starting over with with dry rock and everything is getting quarantined. Hydroids are very, very hard to battle because they are a distant cousin of corals. This means that to kill the hydroids, the coral would die too. Extreme things like bleach, strong concentration of vinegar, things like that will kill the hydroids, but again, semi obviously, the coral will die too. (Second page here has photos from my old system
http://wamas.org/forums/topic/66892-redoing-my-90g-reef/). I talked to Dr. Ron Shimek about this extensively, he said I was screwed. Some, most people they just go away, but for some reason I was a great breeder of them.